DROP TEMPORARY TABLE IF EXISTS tData;
CREATE TEMPORARY TABLE tData
	ENGINE = MEMORY
	SELECT 1 `group`, 1 `max`, 10 `value`
		UNION SELECT 1, 2, 20
		UNION SELECT 1, 3, 30
		UNION SELECT 1, 2, 21
		UNION SELECT 2, '4', 40
		UNION SELECT 2, '6', 60
		UNION SELECT 2, '5', 50
		UNION SELECT 2, '5', 51
		UNION SELECT 3, 7, 70
		UNION SELECT 3, 8, 80
		UNION SELECT 3, 9, NULL
		UNION SELECT 3, 8, 81
		UNION SELECT 4, NULL, 101
		UNION SELECT 4, NULL, 102;

DROP TEMPORARY TABLE IF EXISTS tExpect;
CREATE TEMPORARY TABLE tExpect
	ENGINE = MEMORY
	SELECT 1 `group`, 30 expect
		UNION SELECT 2, 60
		UNION SELECT 3, NULL
		UNION SELECT 4, 102;

SELECT e.`group`, r.result, e.expect, r.result <=> e.expect isRight
	FROM tExpect e
		LEFT JOIN (
			SELECT `group`, multimax(`max`, `value`) result
				FROM tData GROUP BY `group`
		) r ON r.`group` = e.`group`;

DROP TEMPORARY TABLE tData, tExpect;